Translate operators (#335)

This PR adds support for the following operators:
* arithmetic: `+, -, unary +, unary -, *, /, %, ++ prefix and postfix,
-- prefix and postfix`
* relational: `==, !=, >, <, >=, <=, <=>`
* logical: `!, &&, ||`
* bitwise: `~a, &, |, ^, <<, >>`
* assignment: `=, +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=`
* member and pointer: `[] and static [], *, &, ->`
* other: `() and static (), , (comma), user defined conversion`

Not supported:
* `<<` for streams
* `a->*b` pointer to member
* `"a"_b` user defined literal
* `new`, `delete`

In Rust, operators are translated as `operator_call`, `operator_comma`,
etc. Operators that are cv or ref-qualified contain that information in
the name, for example `operator_call_const_volatile_rref`.

We support both method versions and free function versions of operators.
Furthermore, we also support static `operator[]` and static
`operator()`.

The following operators are used to generate Rust traits:
* `operator<` or `operator<=>` generate PartialOrd and Ord
* `operator==` or `operator<` or `operator<=>` are used for PartialEq
and Eq
* the operators are implemented on `Ptr<Self>` while the traits are
implemented on `Self`

I also added rules and tests for `operator<=>`. Since this is a C++20
feature, I had to change CMakeLists, rules/ and Cpp2RustTest.py in order
to compile everything related to `operator<=>` with `-std=c++20`.

Author: lucic71

CMakeLists.txt

@@ -153,11 +153,20 @@ foreach(_rule_dir IN LISTS rule_subdirs)
   get_filename_component(_rule_name ${_rule_dir} NAME)
   set(_out_dir ${RULES_IR_DIR}/${_rule_name})
   set(_out ${_out_dir}/ir_src.json)
+  set(_cxxflags)
+  set(_cxxflags_file)
+  if(EXISTS ${_rule_dir}/cxxflags)
+    set(_cxxflags_file ${_rule_dir}/cxxflags)
+    file(STRINGS ${_cxxflags_file} _flags)
+    foreach(_flag IN LISTS _flags)
+      list(APPEND _cxxflags --cxxflags=${_flag})
+    endforeach()
+  endif()
   add_custom_command(
     OUTPUT ${_out}
     COMMAND ${CMAKE_COMMAND} -E make_directory ${_out_dir}
-    COMMAND $<TARGET_FILE:cpp-rule-preprocessor> --dir ${_rule_dir} --out ${_out}
-    DEPENDS ${_srcs} ${PROJECT_SOURCE_DIR}/cpp2rust/cpp_rule_preprocessor.cpp ${RUSTLS_HEADER}
+    COMMAND $<TARGET_FILE:cpp-rule-preprocessor> --dir ${_rule_dir} --out ${_out} ${_cxxflags}
+    DEPENDS ${_srcs} ${_cxxflags_file} ${PROJECT_SOURCE_DIR}/cpp2rust/cpp_rule_preprocessor.cpp ${RUSTLS_HEADER}
     VERBATIM
   )
   list(APPEND cpp_rules_ir_outputs ${_out})